You can Multiply the whole equation with 2 as it is the denomanot number of 7/2.Therefore,the denomanator will be canceled out with 2 and the number of 7/2 will become 7.
Steps by Steps solution:
C3H8+7/2O2→3CO+4H2OC3H8+72O2→3CO+4H2O
=2×(C3H8+7/2O2→3CO+4H2OC3H8+72O2→3CO+4H2O)
=2C3H8+7O2→6CO+8H2OC3H8+144O2→6CO+8H2O
